Connected: An Internet Encyclopedia
RFC 1812 - 5.2.4.2 Local/Remote Decision

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### 5.2.4.2 Local/Remote Decision

After it has been determined that the IP packet needs to be forwarded according to the rules specified in Section [5.2.3], the following algorithm MUST be used to determine if the Immediate Destination is directly accessible (see [INTERNET:2]).

1. For each network interface that has not been assigned any IP address (the unnumbered lines as described in Section [2.2.7]), compare the router-id of the other end of the line to the IP Destination Address. If they are exactly equal, the packet can be transmitted through this interface.

   DISCUSSION

     In other words, the router or host at the remote end of the line is the destination of the packet or is the next step in the source route of a source routed packet.

2. If no network interface has been selected in the first step, for each IP address assigned to the router:

   1. isolate the network prefix used by the interface.

      IMPLEMENTATION

        The result of this operation will usually have been computed and saved during initialization.

   2. Isolate the corresponding set of bits from the IP Destination Address of the packet.

   3. Compare the resulting network prefixes. If they are equal to each other, the packet can be transmitted through the corresponding network interface.

3. If the destination was neither the router-id of a neighbor on an unnumbered interface nor a member of a directly connected network prefix, the IP Destination is accessible only through some other router. The selection of the router and the next hop IP address is described in Section [5.2.4.3]. In the case of a host that is not also a router, this may be the configured default router.

   Ongoing work in the IETF [ARCH:9, NRHP] considers some cases such as when multiple IP (sub)networks are overlaid on the same link layer network. Barring policy restrictions, hosts and routers using a common link layer network can directly communicate even if they are not in the same IP (sub)network, if there is adequate information present. The Next Hop Routing Protocol (NHRP) enables IP entities to determine the "optimal" link layer address to be used to traverse such a link layer network towards a remote destination.

4. If the selected "next hop" is reachable through an interface configured to use NHRP, then the following additional steps apply:

   1. Compare the IP Destination Address to the destination addresses in the NHRP cache. If the address is in the cache, then send the datagram to the corresponding cached link layer address.
   2. If the address is not in the cache, then construct an NHRP request packet containing the IP Destination Address. This message is sent to the NHRP server configured for that interface. This may be a logically separate process or entity in the router itself.
   3. The NHRP server will respond with the proper link layer address to use to transmit the datagram and subsequent datagrams to the same destination. The system MAY transmit the datagram(s) to the traditional "next hop" router while awaiting the NHRP reply.

Next: 5.2.4.3 Next Hop Address

Connected: An Internet Encyclopedia
RFC 1812 - 5.2.4.2 Local/Remote Decision